1. Field of the Invention
The present invention relates generally to radar systems for determining weather conditions and, more particularly, to an airborne radar system and method for detecting supercooled liquid water drops in the atmosphere.
2. Description of the Prior Art
The task of finding regions within clouds that may present an icing hazard to aircraft requires distinguishing one hydrometeor type from another and estimating their temperature. Hydrometeors are any of the cloud particles composed of liquid water or ice or any of the liquid or ice particles that fall from a cloud as precipitation. In the broadest sense, hydrometeors are therefore of two types, liquid drops and ice crystals.
Liquid water drops form clouds in the atmosphere when the atmosphere becomes saturated with water vapor and it condenses on cloud condensation nuclei. At temperatures between zero and minus forty degrees centigrade, these drops can remain liquid instead of immediately freezing. Liquid water drops at sub-zero temperatures are said to be in a supercooled state. These supercooled liquid water (SLW) drops can grow by continual vapor condensation or by collision and coalescence with one another. SLW drops prevail for minutes to hours, before they eventually evaporate to the vapor phase, freeze to the solid phase, or remain liquid and grow to sufficient size to precipitate.
Clouds, or regions within a cloud, may contain only SLW drops, only ice crystals, or a mixture of both. Because ice crystals are less readily and less abundantly nucleated in the atmosphere than are liquid drops, SLW drops will form more abundantly at all but the coldest temperatures. It is the SLW drops that create the greatest icing hazards. Therefore, when sufficient quantities of SLW drops are in the flight path of an aircraft, the potential for ice accumulation on the aircraft""s wings and other surfaces increases. Ice accumulation can reduce lift, increase drag, and even cause an aircraft to stall or lose control. An average of sixty civilian lives are lost per year due to icing related aircraft accidents.
Weather warning radar systems now in operation generally detect only precipitation sized water drops and ice crystals. Such weather radar systems are usually not sensitive enough to detect liquid drops of drizzle sizes and smaller. Furthermore, because they measure only the echo intensity, they cannot distinguish ice crystals from water crystals, except in selected circumstances by uncertain inferences from other weather information. Additionally, weather radar systems in operation usually do not include the ability to determine the temperature of the drops within a cloud. However, two prior art airborne weather radar systems are known which attempt to distinguish SLW drops. One such system utilizes a single frequency radar system of increased sensitivity and a temperature sensor. Another system uses a temperature sensor and a dual frequency radar system in which the returns of the two frequencies are compared and processed to determine the quantity of SLW. This approach requires the use of two radar systems, one transmitting x-band, and another transmitting Ka-band.
While the prior art radar systems discussed above appear to have had the capability to detect SLW, both ground-based and airborne weather detecting radar systems known to be used at this time are subject to the above mentioned deficiencies. As such, these systems do not distinguish among hydrometeor types to differentiate clouds containing hazardous SLW drops from those clouds which do not. A prior art dual-polarization radar system is known that detects potential storm hazards, but is incapable of SLW drop detection. So, while reports are available for pilots"" use in navigation, the current weather information available to aircraft pilots is of little or no value for avoiding hazardous icing conditions.
It is therefore an object of the present invention to provide an improved icing hazard avoidance system and method.
A further object of this invention is to provide an improved icing hazard avoidance system and method wherein both hydrometeor type and cloud temperature are determined simultaneously.
It is another object of the present invention to provide an improved icing hazard avoidance system and method capable of detecting SLW drops sized somewhat smaller to somewhat larger than drizzle size, which are especially hazardous to aircraft.
It is yet another object of the present invention to provide an improved icing hazard avoidance system and method utilizing a generally conventional radar system and a temperature sensing means.
Briefly, these and other objects may be achieved by an airborne radar system which determines the presence of SLW drops by evaluating the ratio of co-polarized and cross-polarized return signals of a dual-polarization radar in combination with the temperature of the cloud or region of cloud being sensed.